CN112562888B

CN112562888B - Preparation method of silver nanowire solution and preparation method of transparent conductive film

Info

Publication number: CN112562888B
Application number: CN202011478411.3A
Authority: CN
Inventors: 邱业君; 凌建军; 钟留彪; 姜明宇
Original assignee: Shenzhen Graduate School Harbin Institute of Technology
Current assignee: Shenzhen Graduate School Harbin Institute of Technology
Priority date: 2020-12-15
Filing date: 2020-12-15
Publication date: 2022-04-22
Anticipated expiration: 2040-12-15
Also published as: CN112562888A

Abstract

The invention provides a preparation method of a silver nanowire solution and a preparation method of a transparent conductive film, wherein the preparation method of silver nanowires comprises the following steps: step S1, synthesizing silver nanowires by adopting a polyol method, and diluting; and step S2, circularly filtering and purifying the diluted silver nanowires by more than two stages of serially connected tangential flow filter columns, and adding a dispersion modifier for improving the Zeta potential and a ligand exchanger capable of reacting with water to generate active hydrogen in the filtering process. And concentrating the obtained silver nanowire solution, adding a thickening agent and a surfactant, stirring to obtain silver nanowire conductive slurry, coating the silver nanowire conductive slurry on a substrate, and drying to obtain the transparent conductive film. By adopting the technical scheme of the invention, the dispersibility and stability of the silver nanowires are ensured in the purification process, and the effective replacement of the organic adsorbent is realized, so that the transparent conductive film has lower sheet resistance and high sheet resistance uniformity under the conditions of high optical transmittance and low haze.

Description

Preparation method of silver nanowire solution and preparation method of transparent conductive film

Technical Field

The invention belongs to the technical field of preparation of nano photoelectric functional materials, and particularly relates to a preparation method of a silver nanowire solution and a preparation method of a transparent conductive film.

Background

The transparent conductive film is widely applied to display devices, touch devices, transparent heating devices, solar cells, organic light emitting diodes, electromagnetic shielding and other related technical fields as an important photoelectric functional material. With the rapid increase in demand for personal electronic products, the market demand for transparent conductive films has also increased significantly. Currently, Indium Tin Oxide (ITO) has good transmittance (greater than 80%) and low resistivity (10)^-4In the order of Ω/cm). However, ITO faces two problems:

(1) the cost of ITO preparation is high: firstly, in order to obtain the best performance of the ITO, the atomic ratio of indium to tin is 9:1, which means that the mass percent of indium atoms in the ITO is 75%, indium is a rare non-renewable resource, and with the development of the global electronic industry, a large amount of indium resources are consumed, and the price rises dramatically all the way; secondly, the preparation of the ITO requires the use of vacuum evaporation equipment under high temperature conditions, which not only results in high preparation process cost, but also results in failure to achieve optimal performance when deposited on a flexible substrate, and the flexible substrate is heated, curled and deformed, and is prone to generating cracks and other defects, resulting in reduced yield.

(2) ITO, as a metal oxide, is highly brittle and cannot meet the development trend of the flexibility of the current electronic products: the three stages of flexible electronic development are Bendable (Bendable), Foldable (Foldable) and Rollable (Rollable), while ITO is only flexible and has a small curvature, which is prone to brittle fracture.

Aluminum-doped zinc oxide (AZO) and fluorine-doped tin oxide (FTO) are considered suitable inexpensive alternatives to ITO, and are widely used in solar cells. The AZO is generally prepared by a process of annealing after sputtering deposition, the FTO film is prepared by a chemical vapor deposition method, however, the AZO and the FTO have poor stability under high temperature and high humidity conditions, and the etching of the AZO is very difficult, so that the practicability of the AZO in nano/micron application is hindered, and the AZO and the FTO belong to metal oxides, are easy to crack and cannot meet the requirement of flexibility.

In order to solve this problem, various novel transparent conductive film materials have been developed, and most representative materials include conductive polymers, Carbon Nanotubes (CNTs), graphene, metal grids, metal nanowires, and the like. The conductive polymer has low cost, good flexibility and processability, and among them, polyethylenedioxythiophene-poly (styrenesulfonate) (PEDOT: PSS) has been successfully commercialized and widely used in solar cells, nonlinear optical devices, Organic Light Emitting Diodes (OLEDs) and smart windows. However, the conductive polymer has poor chemical stability and unstable doping state, resulting in gradual decrease of conductivity; in the carbon nanotube material, because the contact resistance of the nanotube junction is high, high optical transmittance and low sheet resistance are difficult to realize simultaneously; since the discovery of graphene, with the development of preparation technology, the cost is continuously reduced, but the price is still high, the large-scale production is difficult to realize, and the sheet resistance is high at the time of high transmittance, so that the application of the graphene is limited; the metal grid is a good transparent film-guiding material and has good conductivity, but as the device develops towards micron/nanometer level, the difficulty of depositing the metal grid with nanometer line width is high, and Moire effect exists, so that high-end application is difficult to realize; the metal nanowires used for preparing the transparent conductive film mainly comprise gold nanowires, silver nanowires and copper nanowires, wherein the gold nanowires are expensive, the copper nanowires are poor in conductivity and stability, the silver nanowires have good conductivity, the transmittance of the silver nanowires to light in a visible light region is high, the preparation process is simple, and the like, and the metal nanowires are the best materials for the next-generation transparent conductive film.

The silver nanowire is mainly prepared by a polyol method, and the method has the advantages of high yield, controllable diameter and length of the silver nanowire, low cost and the like. However, non-silver nanowire impurities, such as silver nanoparticles, silver nanorods, silver nanosheets, unreacted silver halide, and the like, inevitably exist in silver nanowires synthesized by the polyol method. These impurities seriously deteriorate the optical properties of the transparent conductive film, resulting in a significant decrease in transmittance and a significant increase in haze, so that purification of silver nanowires is an essential step. At present, the purification method of the silver nanowires mainly comprises centrifugation, negative pressure filtration, positive pressure filtration, acetone flocculation, standing sedimentation, dynamic stirring, centrifugal filtration, tangential flow filtration and the like. These methods all have the problem of silver nanowire agglomeration during the application process, which seriously reduces the filtration efficiency and the silver nanowire yield, and the agglomerated silver nanowires can cause serious deterioration of the optical performance of the transparent conductive film. The surface of the silver nanowire synthesized by adopting a polyalcohol method is generally coated with a layer of polyvinylpyrrolidone (PVP), if the PVP layer cannot be obviously thinned in the purification process, the residual PVP can deteriorate the conductivity of the transparent conductive film, so that the sheet resistance is obviously increased, but the contradiction is that the steric hindrance between the silver nanowires after the PVP is thinned in the purification process is reduced, so that the silver nanowires are easily intertwined to form an irreversible silver nanowire aggregate. Since these aggregates have a large size, they cause severe scattering of light, deteriorating the optical properties of the transparent conductive film.

Disclosure of Invention

Aiming at the technical problems, the invention discloses a preparation method of a silver nanowire solution and a preparation method of a transparent conductive film, which solve the technical problems of silver nanowire agglomeration, poor purification effect, low efficiency, high cost and the like in the existing silver nanowire purification technology, and improve the transmittance of visible light while reducing the haze and the sheet resistance of the prepared conductive film.

In contrast, the technical scheme adopted by the invention is as follows:

a preparation method of silver nanowire solution is characterized by comprising the following steps: it includes:

step S1, synthesizing silver nanowires and diluting;

and step S2, circularly filtering and purifying the diluted silver nanowires by more than two stages of serially connected tangential flow filter columns, and adding a dispersion modifier for improving the Zeta potential and a ligand exchanger capable of reacting with water to generate active hydrogen in the filtering process.

Further, in step S1, silver nanowires are synthesized by a polyol method.

Tangential flow filtration is commonly used for drug purification, water purification, protein separation, cell screening and the like, is a filtration form in which the flow direction of a solution to be filtered and the filtration direction are mutually perpendicular, and has the advantages of high filtration efficiency, difficulty in blocking filter holes and the like compared with the traditional filtration. Tangential flow filtration generally employs hollow fiber filtration membranes, which are composed of a housing containing a circulation inlet/outlet port and two filtrate side ports, and hollow fiber membranes enclosed in the housing. Because the inner diameter of the fiber is very small, the flow pattern of the liquid is in a stable laminar flow state, and the hollow fiber membrane can realize stable filtration performance, higher product recovery rate and linear amplification of treatment scale.

Research shows that the fundamental reasons of pore blocking and photoelectric property deterioration of the conductive film caused by silver nanowire aggregation in the tangential flow purification are as follows: the dispersibility and stability of the silver nanowires in the purification process are deteriorated due to the thinning of the organic adsorption layer, so that irreversible agglomeration occurs, and therefore, the dispersibility and stability of the silver nanowires in the purification process must be improved. There are three main methods for improving the dispersibility and stability of silver nanowires: firstly, a barrier layer is formed on the surface of the silver nanowires, the aggregation of the silver nanowires is prevented through the steric hindrance effect of the barrier layer, the barrier layer is usually realized through polymers adsorbed on the surface of the silver nanowires, and the commonly used polymers are organic adsorption layers with different molecular weights and monomers thereof. Obviously, the method of improving the stability of silver nanowires by a polymeric barrier layer is not desirable in silver nanowire purification, because an important objective in the silver nanowire purification process is to remove the polymeric barrier layer and improve the conductivity of the conductive film. Secondly, improving the dispersibility of the silver nanowires by mechanical stirring and ultrasonic treatment, the method has the limitation that mechanical stirring, although improving the dispersibility of the silver nanowires, cannot disperse irreversible aggregates; the ultrasonic treatment can effectively disperse the silver nanowires, but the problem of excessive local energy generated by the ultrasonic wave can cause the silver nanowires to break, which is also not desirable. And thirdly, the dispersibility and stability of the silver nanowires are improved through electrostatic repulsion, the size of the electrostatic repulsion can be measured by the size of a Zeta potential, and the larger Zeta potential means the larger electrostatic repulsion among the silver nanowires and has higher dispersibility and stability. The pH adjustment of the solution and the surface modification of the silver nanowires are common methods for improving the Zeta potential, but the pH adjustment of the solution is not suitable for the purification process of the silver nanowires, and because a large amount of solvent exchange exists in the purification process and the pH of the solution is difficult to accurately control, the inventor provides a method for improving the Zeta potential by performing surface modification on the silver nanowires to improve the dispersibility and stability of the silver nanowires in the purification process, prevents the agglomeration problem caused by the thinning of the silver nanowires layer by layer due to the organic adsorption layer on the surface, provides possibility for thinning the organic adsorption layer by layer to the maximum extent, reduces the probability of pore blocking of a fiber membrane, and reduces the cost.

Aiming at the problem that the desorption speed of the organic adsorption layer on the surface of the silver nanowire is low in the tangential flow purification process, the inventor proposes that a ligand exchanger solution with a certain concentration is added in the solvent replenishing process so as to improve the desorption speed of the organic adsorption layer. The organic adsorption layer is difficult to desorb through simple cleaning because molecules of the organic adsorption layer contain a large amount of carbonyl groups, and the carbonyl groups and silver atoms on the surface of the silver nanowire form firm Ag-O bonds, and the binding energy of the Ag-O bonds reaches 50.9 kCal/mol. After the ligand exchanger is added into the solution, the ligand exchanger is decomposed to generate a large amount of active hydrogen atoms, and the combination energy of the active hydrogen atoms and the silver atoms on the surface of the silver nanowire to form Ag-H bonds reaches 87.71kCal/mol which is far higher than the Ag-O bonds. Therefore, Ag-O bonds are replaced by Ag-H bonds, and when the concentration of the added ligand exchanger is large enough, the coated organic adsorption layer on the surface of the silver nanowires is completely removed. The added ligand exchanger belongs to electrolyte, and after the ligand exchanger is added, the ion concentration in the solution is increased, the thickness of a double electric layer of the silver nanowire is reduced, the Zeta potential is reduced, and the dispersibility and the stability of the silver nanowire are reduced. Therefore, it is important to control the amount of ligand exchanger added to ensure that silver nanowires do not agglomerate. Meanwhile, the ligand exchanger is decomposed and consumed along with the purification, the desorbed organic adsorption layer is re-adsorbed on the surface of the silver nanowire, and the affinity of the surface modifier selected by the invention to the silver nanowire is between the binding energy of Ag-O bond and Ag-H bond. Therefore, after the ligand exchanger is decomposed and consumed completely, the silver nanowires are preferentially combined with the surface modifier to realize ligand exchange of the silver nanowires, and the dispersibility and stability of the silver nanowires in the process that the organic adsorption layer is quickly thinned in the purification process are guaranteed.

Aiming at the problems of incomplete impurity separation and long circulating filtration time in the process of filtering impurities by using a hollow fiber membrane with a single aperture, the invention provides a method for purifying silver nanowires by adopting a multistage series-connected tangential flow filtration device with different apertures, so that the membrane area of filtration is greatly increased after the hollow fiber membranes are connected in series, the filtration efficiency is improved, and the complete separation of the impurities with different sizes can be realized by reducing the aperture of the fiber membrane step by step.

According to the technical scheme, the dispersion modifier is adopted to modify the surface of the silver nanowires, so that the dispersibility and stability of the silver nanowires in the purification process are improved, the formation of silver nanowire aggregates is prevented, the probability of filter hole blockage in tangential flow filtration is reduced, the filtration efficiency is improved, the service life of the hollow fiber membrane is prolonged, and the cost is reduced. The method has the advantages that the ligand exchanger solution with a certain concentration is added in the tangential flow silver nanowire purification process to accelerate the desorption of the organic adsorption layer on the surface of the silver nanowire layer by layer, and the ligand exchanger solution is matched with a modifier to realize the rapid ligand exchange on the surface of the silver nanowire, namely, the adsorbed ligand on the surface of the silver nanowire is changed from the organic adsorption layer into the modifier, so that the dispersibility and the stability of the silver nanowire are ensured while the organic adsorption layer on the surface of the silver nanowire is effectively thinned. The multistage series tangential flow filtering device is adopted to purify the silver nanowires, the purification efficiency is improved, the purification time cost is reduced, the device can be used for well performing amplification production, and the large-scale purification of the silver nanowires is realized.

In addition, according to the technical scheme, pretreatment of the silver nanowire stock solution synthesized by the polyol method is not required, tangential flow filtration purification can be performed only by diluting the silver nanowire stock solution to a certain concentration range by using a solvent, and reagents harmful to human bodies, such as acetone and the like, are not required in the purification process, so that the process is simple, the cost is low, and the method is green and environment-friendly;

as a further improvement of the present invention, in step S1, an organic adsorbent is attached to the surface of the silver nanowires; the average diameter of the silver nanowires is not more than 100nm, and the average length of the silver nanowires is not less than 5 mu m. Further, the average diameter of the silver nanowires is not more than 50nm, and the average length of the silver nanowires is not less than 10 μm.

As a further improvement of the invention, the organic adsorbent is polyvinylpyrrolidone, and the molecular weight of the polyvinylpyrrolidone is 5000-30000000.

As a further improvement of the invention, the cut-off pore diameter of the tangential flow filtration column is 50 nm-50 μm, and the pore size is reduced with the increase of the number of stages. Furthermore, the interception aperture of the tangential flow filtration column is 50 nm-20 μm.

As a further improvement of the present invention, in step S2, deionized water is used as a solvent during filtration; in step S2, solvent is supplemented in the filtering process, the supplementing speed is 1 ml/min-50L/min, and a dispersion modifier and a ligand exchanger for improving the Zeta potential are added while the solvent is supplemented. Further, in step S2, the solvent is added at a rate of 10ml/min to 5L/min.

The silver nanowire solution enters the hollow fiber of the membrane column, silver nanoparticles and silver nano short rods with small sizes and dissolved organic adsorbent are filtered out from the filtering holes in the wall of the hollow fiber in the solution flowing process, and the silver nanowires with large sizes flow back to the liquid storage bottle along with the liquid flow to be subjected to circulating filtration. The filtration time generally depends on the content of impurities, preferably the filtration time is 1-300 min, more preferably the filtration time is 10-120 min until the filtrate becomes clear, and the solvent is required to be continuously replenished in order to prevent the solution concentration from being too high in the initial stage of filtration. And stopping the supplement of the solvent when the filtrate becomes clear, and continuously filtering the silver nanowire solution which can be concentrated and purified without centrifugal concentration.

As a further improvement of the present invention, the dispersion modifier comprises a positive dispersion modifier and a negative dispersion modifier, the positive dispersion modifier comprises but is not limited to at least one of octadecyl amine hydrochloride, dioctadecyl amine hydrochloride, N-dimethyloctadecyl amine hydrochloride, dodecyltrimethyl ammonium chloride, hexadecyltrimethyl ammonium bromide, octadecyldimethylbenzyl ammonium chloride, and dodecyldimethylphenylphosphonium bromide; the negative electricity dispersion modifier comprises at least one of sodium stearate, N-methyl amide carboxylate, ammonium lauryl sulfate, lauryl alcohol polyoxyethylene ether sodium sulfate, sodium lauryl sulfate, dodecylbenzene sulfonic acid, secondary alkyl sodium sulfonate, polystyrene sodium sulfonate, dodecyl phosphate, polyethylene glycol and sodium polyacrylate.

In the tangential flow circulation filtration purification, a large amount of solvent exchange exists, an organic protective layer (such as polyvinylpyrrolidone) on the surface of the silver nanowires is continuously dissolved in the solvent, the steric hindrance between the silver nanowires is gradually reduced, the silver nanowires are easy to aggregate to form aggregates, and the subsequent effect of the aggregation is that the pore blocking of the hollow fiber membrane is increased and the photoelectric performance of the conductive membrane is deteriorated. The technical scheme of the invention adopts the technical scheme that a dispersion modifier for improving the Zeta potential is added in the filtering process to perform surface modification on the silver nanowires so as to improve the electrostatic repulsion among the silver nanowires, thereby improving the dispersibility and stability of the silver nanowires in the solution. The surface of the silver nanowire is modified by adopting a dispersion modifier, the modifier has characteristic adsorption with the surface of the silver nanowire, and molecules of the modifier enter a Stern layer, so that Stern potential and Zeta potential are greatly improved, and the dispersibility and stability of the silver nanowire in a solution are improved. Furthermore, the size of the Zeta potential can be controlled by controlling the modification amount, so that the dispersibility and stability of the silver nanowire can be regulated and controlled. The dispersion modifying agents are small molecules, and have smaller influence on the conductivity of the silver nanowires compared with organic adsorbents (such as polyvinylpyrrolidone).

As a further improvement of the present invention, the ligand exchanger comprises at least one of borohydride, phosphite, hypophosphite, hypochlorite, and hydrogen peroxide. Further, the body exchanger comprises at least one of sodium borohydride, potassium borohydride and cesium borohydride.

The above mentioned borohydrides can react with water to produce a large number of active hydrogen atoms. The organic adsorption layer on the surface of the silver nanowire has serious influence on the conductivity of the silver nanowire, the desorption and thinning speed of the organic adsorption layer in a tangential flow filtration process is low and incomplete, a ligand exchanger capable of decomposing to generate active hydrogen atoms is added in the tangential flow process, the organic adsorption layer is replaced and removed through Ag-H bonds with high binding energy formed by active hydrogen and the surface of the silver nanowire, and after the active hydrogen relaxation fails, the silver nanowire shows that the binding energy with the dispersion modifier is greater than that of the silver nanowire and the organic adsorbent, and the silver nanowire is preferentially combined with the dispersion modifier, so that the organic adsorbent on the surface is replaced into the dispersion modifier while the dispersibility and stability of the silver nanowire are ensured.

As a further improvement of the method, in step S1, the concentration of the silver nanowire solution obtained by dilution is 0.001-10 wt.%, and then the silver ion complexing agent is added and stirred for 0.001-100 h to remove impurities in the silver halide particles.

Further, in step S1, the concentration of the silver nanowire solution obtained by dilution is 0.01 to 1 wt.%, and then the silver ion complexing agent is added and stirred for 1S to 60min to remove impurities of silver halide particles.

During the process of synthesizing silver nano-wire by adopting polyol methodSilver halide is generated by adding chloride ions and bromide ions and a silver source to promote nucleation of silver and control silver nanowire size, but the silver halide cannot be consumed generally in the reaction process, and the residual final product becomes an impurity. Silver halide is a poorly soluble substance with low solubility in a solvent, and the following dissolution equilibrium AgX ═ Ag exists⁺+X^-(X ═ Cl, Br), adding silver ion complexing agent into the solution to make complex reaction with dissolved silver ion, Ag⁺+xB＝Ag(B)_x ⁺The silver ion complexing agent is mainly salt and organic amine which can ionize and have complexing effect with silver ions, and the like.

Further, the silver ion complexing agent is preferably, but not limited to, one or a mixture of two or more of sodium thiosulfate, ammonia water, ammonium chloride, sodium cyanide, potassium cyanide, ethylene diamine tetraacetic acid, disodium ethylene diamine tetraacetic acid, monoethanolamine, diethanolamine, triethanolamine, tetraethylenepentamine, and diethylenetriamine pentacarboxylate.

Further, the addition amount of the silver ion complexing agent depends on the amount of the silver halide impurities, and the concentration is preferably 0.1 to 5 wt.%, more preferably 0.2 to 3.5 wt.%.

In a further improvement of the present invention, in step S1, the solvent used for dilution is one or a mixture of two or more of water, methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, 1, 2-butanediol, 1, 3-butanediol, n-pentanol, terpineol, ethyl acetate, propyl acetate, butyl acetate, dimethylformamide, and tetrahydrofuran. More preferably, the concentration of the silver nanowires in the solution is 0.001-0.7 wt.%.

As a further improvement of the invention, in step S2, more than two stages of tangential flow filter columns connected in series are adopted to circularly filter and purify the diluted silver nanowires, and the time of circular filtration is 0.001-200 h. Further, the time of circulating filtration is 1-300 min.

The invention also discloses a preparation method of the transparent conductive film, which comprises the following steps:

preparing the silver nanowire solution by adopting the preparation method of the silver nanowire solution;

concentrating the filtered and purified silver nanowire solution to enable the concentration of the silver nanowires to be 0.002-10 wt.%; further, the concentration of the concentrated silver nanowires is 0.01-1 wt.%;

adding a thickening agent and a surfactant into the concentrated silver nanowire solution, and stirring to obtain silver nanowire conductive slurry;

and coating the silver nanowire conductive paste on a substrate, and drying to obtain the transparent conductive film.

The thickening agent is used for increasing the viscosity of the slurry and promoting the silver nanowire to form a film, the silver nanowire network is wrapped after the silver nanowire network is dried, the adhesive force between the silver nanowire network and a base material is increased, and meanwhile a certain protection effect is achieved on the silver nanowire layer. The surfactant is used for reducing the surface tension of the slurry, improving the wettability and the leveling property of the slurry to a base material and improving the coating uniformity. Thickeners include common celluloses, preferably but not limited to carboxy cellulose, ethyl cellulose, propyl cellulose, hydroxy cellulose, hydroxypropyl methyl cellulose, and the like. The concentration of the thickener is 0.01-1 wt%, preferably 0.05-0.75 wt%. Suitable surfactants include, but are not limited to, ZONYL DuPont aqueous ethoxy nonionic fluorocarbon surfactants, Triton polyethylene glycol octylphenyl ether nonionic surfactants, and the like. Preferred surfactants are Triton X100, Triton X45, ZONYL FSO-100, ZONYL FS-300, and the like. The concentration of the surfactant is 0.001-0.1 wt.%, preferably 0.005-0.05 wt.%.

The silver nanowire conductive paste is coated on a base material by a method such as blade coating, wire rod coating, roll-to-roll slit coating and the like. Substrates include, but are not limited to, polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), Polyimide (PI), polyamide, Polyetheretherketone (PEEK), Polyethersulfone (PES), Polyetherimide (PEI), and the like. The thickness of the applied wet film is 2 to 200. mu.m, and preferably, the thickness of the applied wet film is 5 to 50 μm.

The drying is to remove the solvent in the wet film and leave the silver nanowire conductive network on the substrate, and infrared heating drying, oven heating drying and the like can be adopted, and the silver nanowire conductive network can also be naturally air-dried at room temperature. The drying temperature is 25-300 ℃, preferably 80-250 ℃, and more preferably 100-180 ℃. The drying time is 1s-30min, preferably 10 s-20 min, more preferably 20 s-15 min.

By adopting the technical scheme, the silver nanowire conductive paste or the silver nanowire conductive ink for coating preparation can be prepared by directly concentrating the purified tangential flow of the silver nanowires to a certain concentration and then adding a certain amount of thickening agent and surfactant into the purified tangential flow of the silver nanowires, so that the integration of the purification of the silver nanowires and the preparation of the conductive ink is realized, and the process flow is further simplified. In the obtained transparent conductive film, the dispersion modifier improves the electrostatic repulsion among the silver nano-wires, so that the silver nano-wires are uniformly distributed on the transparent conductive film. Thereby the transparent conductive film has lower sheet resistance and high sheet resistance uniformity under the conditions of high optical transmittance and low haze.

Compared with the prior art, the invention has the beneficial effects that:

by adopting the technical scheme of the invention, the diluted silver nanowires are filtered by adopting a multistage series tangential flow filter column combination device, and in the filtering process, a dispersion modifier for improving the Zeta potential is added to improve the dispersibility and stability of the silver nanowires; the ligand exchanger is added to promote the desorption of the organic adsorption layer on the surface of the silver nanowire, the obtained silver nanowire is high in purification efficiency, good in effect and low in cost and is suitable for industrial production, the dispersibility and stability of the silver nanowire are guaranteed in the purification process, meanwhile, the organic adsorbent is effectively replaced, the prepared silver nanowire transparent conductive film is excellent in performance, the dispersion modifier improves the electrostatic repulsion among the silver nanowires, the silver nanowires are uniformly distributed on the transparent conductive film, softening occurs in the drying process of the conductive film to promote the direct contact among the silver nanowires, so that the sheet resistance of the film is reduced, and the transparent conductive film has lower sheet resistance and high sheet resistance uniformity under the conditions of high optical transmittance and low haze.

Drawings

FIG. 1 is a schematic diagram of a multistage series tangential flow filtration column assembly of the present invention.

FIG. 2 is a schematic of tangential flow filtration according to the present invention.

FIG. 3 is a schematic representation of the Stern bilayer theory of the present invention, wherein a) is a schematic diagram of the electric double layer of colloidal particles and b) is a histogram.

FIG. 4 is a graph showing the potential distribution after modification of colloidal particles with a gesture modifier of the same sign or different sign according to the present invention, wherein a) is the potential distribution after modification of colloidal particles with a modifier of different sign, and b) is the potential distribution after modification of colloidal particles with a modifier of the same sign.

FIG. 5 is a schematic representation of the ligand exchange achieved by the addition of a ligand exchanger during tangential flow filtration according to the present invention.

Fig. 6 is SEM images of the silver nanowire stock solution and the purified concentrate according to example 1 of the present invention, wherein the left image is an SEM image of the silver nanowire stock solution, and the right image is an SEM image of the purified concentrate.

Fig. 7 is SEM images of silver nanowire stock solution and purified concentrate according to embodiment 2 of the present invention, wherein the left image is SEM image of silver nanowire stock solution, and the right image is SEM image of purified concentrate.

Fig. 8 is SEM images of the silver nanowire stock solution and the purified concentrate according to example 3 of the present invention, wherein the left image is an SEM image of the silver nanowire stock solution, and the right image is an SEM image of the purified concentrate.

Fig. 9 is SEM images of the silver nanowire stock solution and the purified concentrate according to example 4 of the present invention, wherein the left image is an SEM image of the silver nanowire stock solution, and the right image is an SEM image of the purified concentrate.

Fig. 10 is SEM images of a silver nanowire stock solution and a purified concentrate according to comparative example 5 of the present invention, wherein the left image is an SEM image of the silver nanowire stock solution, and the right image is an SEM image of the purified concentrate.

Fig. 11 is SEM images of a silver nanowire stock solution and a purified concentrate of comparative example 6 of the present invention, wherein the left image is an SEM image of the silver nanowire stock solution and the right image is an SEM image of the purified concentrate.

Fig. 12 is a high-magnification SEM image of the silver nanowire stock solution and the purified concentrate in example 3 of the present invention, wherein the left image is an SEM image of the silver nanowire stock solution, and the right image is an SEM image of the purified concentrate.

The reference numerals include:

1-liquid storage bottle, 2-magnetic stirrer, 3-peristaltic pump, 4-hollow fiber membrane filter column, 5-waste liquid bottle, 6-liquid supplement pipe, 7-liquid inlet pipe, 8-waste liquid pipe, 9-liquid outlet pipe and 10-reflux pipe.

Detailed Description

Preferred embodiments of the present invention are described in further detail below.

The invention will be further illustrated with reference to the following specific examples. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention. The experimental procedures, in which specific conditions are not noted in the following examples, are generally carried out according to conventional conditions or according to conditions recommended by the manufacturers.

A preparation method of a transparent conductive film is provided, the transparent conductive film is composed of a transparent substrate, a silver nanowire conductive network and a resin protective layer, wherein the preparation method of the silver nanowire comprises the following steps:

(1) synthesizing silver nanowires by adopting a polyol method;

the silver nanowires are the most important component of the transparent conductive film, and the photoelectric property of the transparent conductive film is determined by the size of the silver nanowires. The longer the silver nanowires are, the higher the probability of mutual lap joint among the silver nanowires is, so that a conductive network is formed to reduce the sheet resistance; the smaller the diameter of the silver nanowire is, the smaller the scattering of light is, so that the light transmittance can be improved, and the haze can be reduced. In order to make the conductive film have good light transmittance and low haze under the condition of certain sheet resistance, the thin and long silver nanowires are used. In particular, silver nanowires having an average diameter of less than 50nm and an average length of more than 10 μm are preferred. More preferably, the silver nanowires have an average diameter of less than 35nm and an average aspect ratio of greater than 500.

Silver nanowires can be prepared by methods known in the art. Specifically, silver nanowires can be prepared by colloidal reduction of a silver salt (e.g., silver nitrate, silver chloride, and silver bromide) in the presence of a polyol (e.g., ethylene glycol, propylene glycol, glycerol, or the like) and polyvinylpyrrolidone, resulting in a silver nanowire stock solution.

In the prior art of preparing silver nanowires by polyol colloidal reduction, polyvinylpyrrolidone is used as an organic adsorbent, which is used for rapidly adsorbing silver salt on the surface of metallic silver to inhibit silver atoms from depositing in a certain direction when the silver salt is reduced into silver simple substance, so that the silver nanowires grow into a one-dimensional nanowire structure. Meanwhile, the organic adsorbent is adsorbed on the surface of the silver nanowires, so that steric hindrance is formed among the silver nanowires, the dispersity of the silver nanowires is improved, and the silver nanowires are prevented from being oxidized. However, the presence of the organic adsorbent seriously affects the conductivity of the silver nanowire conductive network formed on the transparent conductive film because the organic adsorbent prevents direct contact of silver between the silver nanowires, and electrons can only be conducted by tunneling. The inventor finds that the junction resistance between silver nanowires is reduced along with the reduction of the thickness of the organic adsorption layer on the surfaces of the silver nanowires when the silver nanowires with the average diameter of 26nm and the average length of 26 μm are used for preparing the transparent conductive film. As a result, the sheet resistance of the transparent conductive film is significantly reduced under the same haze and visible light transmittance.

(2) Purification of silver nanowires

The silver nanowire stock solution prepared by the polyol method often contains non-silver nanowire impurities, such as silver halide particles, silver nanoparticles, silver nanorods and the like which are not completely reacted. These impurities not only do not contribute to the conduction of the network of the conductive film, but also severely increase the scattering of light, so that the haze of the conductive film is significantly increased and the visible light transmittance is significantly reduced. In order to provide excellent photoelectric properties to the transparent conductive film, it is necessary to purify the silver nanowires. The purification purpose is two, one is to remove silver halide particles, silver nanoparticles and silver nano short rods in the synthesized silver nanowires; and secondly, removing or replacing the organic adsorbent part on the surface of the silver nanowire.

The purification of the silver nanowires is completed by tangential flow circulation filtration, and in order to improve the filtration efficiency, the invention adopts a multistage series tangential flow filtration device to filter and purify the silver nanowires. The device at least comprises two stages of tangential flow filtration column filtration devices, as shown in figure 1, each stage of tangential flow filtration device comprises a liquid storage bottle 1, a magnetic stirrer 2, a peristaltic pump 3, a hollow fiber membrane filtration column 4, a waste liquid bottle 5, a liquid supplementing pipe 6, a liquid inlet pipe 7, a waste liquid pipe 8, a liquid outlet pipe 9 and a return pipe 10. The interception aperture of the hollow fiber membrane filtration column 4 is gradually reduced, and the hollow fiber membrane filtration column has the function of removing impurities such as silver nano short rods, silver nano short wires, silver nano particles and the like in the silver nano wire solution.

Further, the silver nanowire purification and slurry treatment are completed according to the following steps:

step S11, diluting the silver nanowire stock solution with a solvent until the solid content reaches a certain concentration range, wherein the preferable concentration is 0.001-0.7 wt.%; solvents used to dilute the silver nanowire stock solution include, but are not limited to: water, ethanol, methanol, ethyl acetate, propyl acetate, butyl acetate, isopropyl alcohol and the like, and a mixed solution thereof, and more preferably, the concentration of the diluted silver nanowires is 0.003 to 0.5 wt.%.

Step S12, adding a certain amount of silver ion complexing agent into the silver nanowire solution diluted to a certain concentration in the step S11, and stirring for 1S-60min to remove impurities of silver halide particles; the silver ion complexing agent is preferably, but not limited to, sodium thiosulfate, ammonia, ammonium chloride, sodium cyanide, potassium cyanide, ethylenediaminetetraacetic acid, disodium ethylenediaminetetraacetate, monoethanolamine, diethanolamine, triethanolamine, tetraethylenepentamine, diethylenetriaminepentacarboxylate, and the like, and mixtures of two or more thereof. The addition amount of the silver ion complexing agent depends on the amount of silver halide impurities, and the concentration of the silver ion complexing agent is preferably 0.1-5 wt.%, and more preferably 0.2-3.5 wt.%.

S13, performing tangential flow circulating filtration and purification on the silver nanowire solution processed in the step S12, adding a dispersion modifier and a ligand exchanger which improve the Zeta potential to promote the replacement of an organic adsorption layer on the surface of the silver nanowire while replenishing the solvent in the filtration process, and simultaneously ensuring the dispersibility and stability of the silver nanowire, wherein the circulating filtration time is 1-300 min;

step S14, after the filtration and purification in step S13, the purified silver nanowire solution is concentrated by using a last-stage tangential flow device to enable the concentration of the silver nanowire solution to be in a certain range, and the concentration of the preferred silver nanowire solution after concentration is 0.01-1 wt.%.

Fig. 2 is a schematic diagram of tangential flow filtration purification of silver nanowires, when a silver nanowire solution enters hollow fibers of a membrane column, silver nanoparticles and silver nano short rods with smaller sizes and dissolved organic adsorbents are filtered out from filter holes on the wall of the hollow fibers in the solution flowing process, and silver nanowires with larger sizes are circulated and filtered along with liquid flow flowing back to a liquid storage bottle. The filtration time generally depends on the content of impurities, preferably the filtration time is 1-300 min, more preferably the filtration time is 10-120 min until the filtrate becomes clear, and the solvent is required to be continuously replenished in order to prevent the solution concentration from being too high in the initial stage of filtration. And stopping the supplement of the solvent when the filtrate becomes clear, and continuously filtering the silver nanowire solution which can be concentrated and purified without centrifugal concentration.

In the tangential flow circulation filtration purification, a large amount of solvent exchange exists, an organic protective layer (such as polyvinylpyrrolidone) on the surface of the silver nanowires is continuously dissolved in the solvent, the steric hindrance between the silver nanowires is gradually reduced, the silver nanowires are easy to aggregate to form aggregates, and the subsequent effect is that the pore blocking of the hollow fiber membrane is increased and the photoelectric performance of the conductive membrane is deteriorated. The invention provides a method for modifying the surface of silver nanowires by using a dispersion modifier so as to improve electrostatic repulsion among the silver nanowires, thereby improving the dispersibility and stability of the silver nanowires in a solution. The principle of this is known from the classical DLVO theory (wherein DLVO theory was proposed by delayagar (Derjguin) and randau (Landau) in 1941, respectively, and french (Verwey) and ovbek (overture) in 1948), the stability of colloidal nanoparticles mainly depends on the interaction between particles, and the total force is expressed as the sum of van der waals attractive force and electrostatic repulsive force due to an electric double layer. The effective diameter of the colloidal nanoparticles is equal to the size of the colloidal nanoparticle check plus twice the thickness of the double diffusion layers of the colloidal nanoparticles. Therefore, the distance between the colloidal nanoparticles can be regulated and controlled by regulating and controlling the thickness of the double electric layers of the colloidal nanoparticles, and the dispersity and the stability of the colloidal nanoparticles can be further regulated and controlled. According to the model of Stern double electric layer, the adsorbed colloidal particles are shown in FIG. 3The ions on the surface can be divided into two layers, wherein the first layer is the ions which are adsorbed on the surface of the particles through the characteristic and is called a Stern layer; the second layer is the ions attracted by electrostatic forces and is called the diffusion layer. When the particles move, not only the ions in the Stern layer move with the particles, but also the ions in a layer of solution outside the Stern layer move with the particles, and the outer surface of the solution layer is called as a sliding surface. Potential distribution in the double electric layer, in the Stern layer, by the surface potential Ψ₀Linearly down to Stern potential Ψ_δThe electric potential on the sliding surface is called Zeta potential, which is an important index for characterizing the stability of colloid, and the decrease of the Ztea electric potential reduces the electrostatic repulsion between particles, and van der waals attraction dominates, thereby causing the coagulation and destruction of colloid.

When the dispersion modifier is used for surface modification of the silver nanowires, the modification agent and the surfaces of the silver nanowires have characteristic adsorption effect, and modifier molecules enter a Stern layer, so that Stern potential and ZTea potential are greatly improved, and the dispersibility and stability of the silver nanowires in a solution are improved. The dispersion modifier and the ion surface ions with the same sign and different sign charges are provided, the potential distribution of the modified dispersion modifier and the ion surface ions is shown in figure 4, the magnitude of the Zeta potential can be controlled through the modification amount, and the dispersibility and the stability of the silver nanowire are further regulated and controlled.

The dispersion modifying agent used in the present invention can be classified into two types, a positive dispersion modifying agent and a negative dispersion modifying agent. The positive electricity dispersion modifier is mainly primary secondary tertiary amine salt and quaternary ammonium salt, preferably but not limited to octadecyl amine hydrochloride, dioctadecyl amine hydrochloride, N-dimethyl octadecyl amine hydrochloride, dodecyl trimethyl ammonium chloride, hexadecyl trimethyl ammonium bromide, octadecyl dimethyl benzyl ammonium chloride, dodecyl dimethyl phenyl phosphorus bromide and a mixture of two or more of the above. The negative electricity dispersion modifier is mainly carboxylate, sulfate salt, sulfonate and phosphate salt, preferably but not limited to sodium stearate, N-methyl amide carboxylate, ammonium lauryl sulfate, lauryl alcohol polyoxyethylene ether sodium sulfate, sodium lauryl sulfate, dodecylbenzene sulfonic acid, secondary alkyl sodium sulfonate, polystyrene sodium sulfonate, dodecyl phosphate, polyethylene glycol, sodium polyacrylate, and mixtures of two or more of them.

The organic adsorption layer on the surface of the silver nanowire has serious influence on the conductivity of the silver nanowire, the desorption and thinning speed of the organic adsorption layer in the tangential flow filtration process is low and incomplete, the ligand exchanger capable of decomposing to generate active hydrogen atoms is added in the tangential flow process, the schematic diagram is shown in figure 5, the organic adsorption layer is replaced and removed through Ag-H bonds with high binding energy formed by active hydrogen and the surface of the silver nanowire, and after the relaxation of the active hydrogen fails, the organic adsorption layer is preferentially combined with a dispersion modifier because the binding energy of the silver nanowire and the dispersion modifier is larger than the binding energy of the silver nanowire and the organic adsorbent, so that the dispersibility and the stability of the silver nanowire are ensured, and the organic adsorbent on the surface is replaced by the dispersion modifier. The dispersion modifying agents are small molecules, and have smaller influence on the conductivity of the silver nanowires compared with organic adsorbents (such as polyvinylpyrrolidone). The ligand exchanger used in the present invention is mainly a compound containing boron hydride, preferably but not limited to sodium borohydride, potassium borohydride, cesium borohydride and a mixture of two or more thereof, etc.

Adding a certain amount of thickening agent and surfactant into the concentrated silver nanowire solution, and stirring to obtain the silver nanowire conductive paste for coating the transparent conductive film. The thickener has the functions of increasing the viscosity of the slurry, promoting the film formation of the silver nanowires, wrapping the silver nanowire network after drying, increasing the adhesive force with the base material and simultaneously playing a certain role in protecting the silver nanowire layer. The surfactant is used for reducing the surface tension of the slurry, improving the wettability and the leveling property of the slurry to a base material and improving the coating uniformity. Thickeners include the common celluloses. Preferred are, but not limited to, carboxy cellulose, ethyl cellulose, propyl cellulose, hydroxy cellulose, hydroxypropyl methyl cellulose, and the like. The concentration of the thickener is 0.01-1 wt%, preferably 0.05-0.75 wt%. Suitable surfactants include, but are not limited to, ZONYL DuPont aqueous ethoxy nonionic fluorocarbon surfactants, Triton polyethylene glycol octylphenyl ether nonionic surfactants, and the like. Preferred surfactants are Triton X100, Triton X45, ZONYL FSO-100, ZONYL FS-300, and the like. The concentration of the surfactant is 0.001-0.1 wt.%, preferably 0.005-0.05 wt.%.

After the silver nanowire conductive paste is obtained, the silver nanowire conductive paste can be coated by methods such as blade coating, wire rod coating, roll-to-roll slit coating and the like. Coated substrates include, but are not limited to, polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), Polyimide (PI), polyamide, Polyetheretherketone (PEEK), Polyethersulfone (PES), Polyetherimide (PEI), and the like. The wet film thickness of the coating is 2 to 200. mu.m, and the preferred coating thickness is 5 to 50 μm.

After coating, the coating is dried to obtain a transparent conductive film. The drying is to remove the solvent in the wet film and leave the silver nanowire conductive network on the substrate, and infrared heating drying, oven heating drying and the like can be adopted, and the silver nanowire conductive network can also be naturally air-dried at room temperature. The drying temperature is 25-300 ℃, preferably 80-250 ℃, and more preferably 100-180 ℃. The drying time is 1s-30min, preferably 10 s-20 min, more preferably 20 s-15 min.

The following description will be given with reference to specific examples.

Example 1

(1) Silver nanowire synthesis

Ethylene glycol was prepared as a solvent and a reducing agent, silver nitrate as a silver source, polyvinylpyrrolidone having a molecular weight of 1300000 as an organic adsorbent, and sodium chloride and sodium bromide as control agents.

First, three different ethylene glycol solutions a were prepared: 220.0mM NaBr, solution B: 210.0mM NaCl, solution C: 50.5mM polyvinylpyrrolidone, a 2.5L single-neck flask is taken, 18.2mL of solution A, 24mL of solution B and 2L of solution C are added, a turning-over plug is used for plugging and stirring, and 10g of AgNO is added after uniform mixing₃Stirring in dark until the solution is dissolved, then placing the flask in an oil bath at 170 ℃ for reaction for 1.5h, taking out the flask after the reaction is finished, and placing the flask in ice water for quenching to obtain silver nanowire stock solution for later use. Diluting 0.2mL of the stock solution with ethanol to 1.5mL, centrifuging at 6000rpm, and repeatingThe silver nanowires were dispersed three times and dropped on a silicon wafer to be characterized by a high resolution field emission scanning electron microscope S-4700, as shown in fig. 6, Image J software was used to measure and count the diameters and lengths of the silver nanowires in a plurality of scanning electron microscope photographs, and the result shows that the average diameter of the silver nanowires prepared in this example is about 36nm, and the average length is about 35 μm.

(2) Silver nanowire purification

Dispersing 2L of silver nanowire stock solution into 6L of deionized water, adding 200mL of ammonia water serving as a silver ion complexing agent, and stirring for 10min at the stirring speed of 500 rpm. Filtering the silver nanowire dispersion liquid by using a series three-stage tangential flow filtering device, wherein the interception pore diameters of filtering columns are 1.5 microns, 1 micron and 0.5 micron respectively, adding deionized water into a liquid storage tank while performing circulating filtration, wherein the adding speed is 100mL/min, and adding a dispersion modifier while adding deionized water after filtering for 10min, wherein the dispersion modifier added in the embodiment is sodium polyacrylate, the concentration of the dispersion modifier is 10 wt%, and 100mL is added at one time. After 20min of the loop filtration, the ligand exchanger was added with additional deionized water, in this example sodium borohydride at a concentration of 10 wt.%, and 40mL was added by syringe pump at a rate of 4 mL/min. And then, continuously performing tangential flow circulation filtration for 40min to complete the purification of the silver nanowire stock solution, adding 50mL of 10 wt.% sodium polyacrylate into the purified solution, and stirring for 2h at 500rpm to perform secondary modification on the silver nanowires, thereby further improving the dispersibility and stability of the silver nanowires. After finishing the modification, after the concentration of the purified silver nanowires is measured by adopting a drying mode, the silver nanowires are subjected to tangential flow filtration and concentration, and the silver nanowires are concentrated by adopting a last-stage tangential flow system, wherein the concentration of the concentrated silver nanowires is in a proper range of 0.15-0.3 wt.%, so that a silver nanowire concentrated solution is obtained.

(3) Slurrying

Preparing silver nanowire conductive paste: the conductive paste formulation of this example was 0.1 wt.% silver nanowire concentration, 0.3 wt.% thickener hydroxyethyl methylcellulose concentration, and 0.015 wt.% surfactant Triton X100 concentration.

(4) Preparation of transparent conductive film

The silver nanowire conductive paste was coated on a PET film substrate of 148mm × 210mm using a blade coater, and a film of each thickness was formed by adjusting the gap between the blade and the substrate. And drying the coated film at 130 ℃ for 3min to obtain the silver nanowire transparent conductive film.

(5) Measurement of photoelectric Properties of transparent conductive films

The sheet resistance of each transparent conductive film was measured using an RTS-9 type four-probe sheet resistance meter manufactured by guangzhou four-probe technology ltd, and the sheet resistances of 9 different regions were measured, and the average value thereof was taken as the average sheet resistance. Transmittance and haze of the transparent conductive film were measured simultaneously using a WGT-S type transmittance haze meter, shanghai optical spectrometer, ltd, measuring transmittance and haze of 9 different regions, and taking the average value thereof as average transmittance and haze.

The main experimental results are shown in table 1.

Example 2

(1) Silver nanowire synthesis

First, three different ethylene glycol solutions a were prepared: 220.0mM NaBr, solution B: 210.0mM NaCl, solution C: 50.5mM polyvinylpyrrolidone, a 2.5L single-neck flask is taken, 13.6mL of solution A, 24mL of solution B and 2L of solution C are added, a turning-over plug is used for plugging and stirring, and 10g of AgNO is added after uniform mixing₃Stirring in dark until the solution is dissolved, then placing the flask in an oil bath at 170 ℃ for reaction for 1.5h, taking out the flask after the reaction is finished, and placing the flask in ice water for quenching to obtain silver nanowire stock solution for later use. Diluting 0.2mL of the stock solution with ethylene to 1.5mL, centrifuging and re-dispersing for three times at 6000rpm, dropwise adding the stock solution on a silicon wafer, representing the stock solution through a high-resolution field emission scanning electron microscope S-4700, and measuring and counting the diameters and the lengths of the silver nanowires in a plurality of scanning electron microscope photos by using Image J software, as shown in FIG. 7, the result shows that the silver nanowires prepared in the example are subjected to measurement and statisticsThe average diameter was about 30nm and the average length was about 31 μm.

Silver nanowire purification, slurry formation, transparent conductive film preparation, and measurement of photoelectric properties of the transparent conductive film were performed under the same conditions as in example 1.

The main experimental results are shown in table 1.

Example 3

(1) Silver nanowire synthesis

First, three different ethylene glycol solutions a were prepared: 220.0mM NaBr, solution B: 210.0mM NaCl, solution C: 50.5mM polyvinylpyrrolidone, a 2.5L single-neck flask is taken, 18.2mL of solution A, 24mL of solution B and 2L of solution C are added, a turning-over plug is used for plugging and stirring, and 10g of AgNO is added after uniform mixing₃Stirring in dark until the solution is dissolved, then placing the flask in an oil bath at 170 ℃ for reaction for 1.5h, taking out the flask after the reaction is finished, and placing the flask in ice water for quenching to obtain silver nanowire stock solution for later use. 0.2mL of the stock solution is diluted to 1.5mL by using the ethyl, centrifuged and re-dispersed three times at 6000rpm, and the diluted stock solution is dripped on a silicon wafer to be characterized by a high-resolution field emission scanning electron microscope S-4700, and the diameters and the lengths of the silver nanowires in a plurality of scanning electron microscope photos are measured and counted by using Image J software, as shown in FIGS. 8 and 12, the result shows that the average diameter of the silver nanowires prepared in the example is about 25nm, and the average length of the silver nanowires is about 26 μm.

(2) Silver nanowire purification

Dispersing 2L of silver nanowire stock solution into 6L of deionized water, adding 200mL of ammonia water serving as a silver ion complexing agent, and stirring for 10min at the stirring speed of 500 rpm. Filtering the silver nanowire dispersion liquid by using a series three-stage tangential flow filtering device, wherein the interception pore diameters of filtering columns are 0.8 mu m, 0.5 mu m and 0.3 mu m respectively, adding deionized water into a liquid storage tank while performing circulating filtration, wherein the replenishment speed is 100mL/min, and after filtering for 30min, adding a dispersion modifier while replenishing the deionized water, wherein the dispersion modifier added in the example is sodium polyacrylate, the concentration of the dispersion modifier is 10 wt%, and 100mL is added at one time. After 20min of the loop filtration, the ligand exchanger was added with additional deionized water, in this example sodium borohydride at a concentration of 10 wt.%, and 40mL was added by syringe pump at a rate of 4 mL/min. And then, continuously performing tangential flow circulating filtration for 60min to complete the purification of the silver nanowire stock solution, adding 50mL of 10 wt.% sodium polyacrylate into the purified solution, and stirring for 2h at 500rpm to perform secondary modification on the silver nanowires, thereby further improving the dispersibility and stability of the silver nanowires. After finishing the modification, after the concentration of the purified silver nanowires is measured by adopting a drying mode, the silver nanowires are subjected to tangential flow filtration and concentration, and the silver nanowires are concentrated by adopting a last-stage tangential flow system, wherein the concentration of the concentrated silver nanowires is in a proper range of 0.15-0.3 wt.%, so that a silver nanowire concentrated solution is obtained.

The slurry formation, the preparation of the transparent conductive film, and the measurement of the photoelectric properties of the transparent conductive film were performed under the same conditions as in example 1.

The main experimental results are shown in table 1.

Example 4

(1) Silver nanowire synthesis

First, three different ethylene glycol solutions a were prepared: 220.0mM NaBr, solution B: 210.0mM NaCl, solution C: 50.5mM polyvinylpyrrolidone, a 2.5L single-neck flask is taken, 18.2mL of solution A, 24mL of solution B and 2L of solution C are added, a turning-over plug is used for plugging and stirring, and 10g of AgNO is added after uniform mixing₃Stirring in dark until the solution is semitransparent milky white, adding 15g of benzoin, stirring for 10min, placing the flask in a 160 ℃ oil bath for reaction for 1.5h, taking out the flask after the reaction is finished, and placing the flask in ice water for quenching to obtain silver nanowire stock solution for later use. Diluting the stock solution 0.2mL with ethanol to 1.5mL at 6000rpmThe solution is centrifuged and re-dispersed three times, and is dripped on a silicon wafer to be characterized by a high-resolution field emission scanning electron microscope S-4700, and the diameters and the lengths of the silver nanowires in a plurality of scanning electron microscope photos are measured and counted by using Image J software, as shown in FIG. 9, the result shows that the average diameter of the silver nanowires prepared by the example is about 20nm, and the average length is about 19 μm.

The purification of silver nanowires, slurrying, preparation of transparent conductive films, and measurement of photoelectric properties of the transparent conductive films were performed under the same conditions as in example 3.

The main experimental results are shown in table 1.

Example 5

Silver nanowire synthesis was performed using the same conditions as in example 3.

Purification of silver nanowires

Dispersing 2L of silver nanowire stock solution into 6L of deionized water, adding 170mL of tetraethylenepentamine serving as a silver ion complexing agent, and stirring for 10min at the stirring speed of 500 rpm. Filtering the silver nanowire dispersion liquid by using a series three-stage tangential flow filtering device, wherein the interception pore diameters of filtering columns are 0.8 mu m, 0.5 mu m and 0.3 mu m respectively, adding deionized water into a liquid storage tank while performing circulating filtration at the replenishment speed of 10mL/min, and after filtering for 10min, adding a dispersion modifier while replenishing the deionized water, wherein the dispersion modifier added in the example is dodecylbenzene sulfonic acid, the concentration of the dodecylbenzene sulfonic acid is 15 wt%, and 80mL is added at one time. After 25min of the loop filtration, the ligand exchanger was added with additional deionized water, in this example potassium borohydride at a concentration of 10 wt.%, and 40mL was added by syringe pump at a rate of 4 mL/min. And then, continuously performing tangential flow circulation filtration for 40min to complete the purification of the silver nanowire stock solution, adding 30mL of dodecyl benzene sulfonic acid with the concentration of 15 wt.% into the purified solution, and stirring for 2h at 500rpm to perform secondary modification on the silver nanowires, thereby further improving the dispersibility and the stability of the silver nanowires. After finishing the modification, after the concentration of the purified silver nanowires is measured by adopting a drying mode, the silver nanowires are subjected to tangential flow filtration and concentration, and the silver nanowires are concentrated by adopting a last-stage tangential flow system, wherein the concentration of the concentrated silver nanowires is in a proper range of 0.15-0.3 wt.%, so that a silver nanowire concentrated solution is obtained.

The slurry formation and the transparent conductive film production were carried out under the same conditions as in example 1 to measure the photoelectric properties of the transparent conductive film.

The main experimental results are shown in table 1.

Example 6

Purification of silver nanowires

Dispersing 2L of silver nanowire stock solution into 6L of deionized water, adding 170mL of tetraethylenepentamine serving as a silver ion complexing agent, and stirring for 10min at the stirring speed of 500 rpm. Filtering the silver nanowire dispersion liquid by using a series three-stage tangential flow filtering device, wherein the interception pore diameters of a filtering column are 0.8 mu m, 0.5 mu m and 0.3 mu m respectively, adding deionized water into a liquid storage tank while performing circulating filtration, wherein the replenishment speed is 10mL/min, and after filtering for 10min, adding a dispersion modifier while replenishing the deionized water, wherein the dispersion modifier added in the example is octadecyl dimethyl benzyl ammonium chloride, the concentration of the octadecyl dimethyl benzyl ammonium chloride is 30 wt.%, and 25mL is added at one time. After 25min of the loop filtration, the ligand exchanger was added with additional deionized water, in this example potassium borohydride at a concentration of 10 wt.%, and 40mL was added by syringe pump at a rate of 4 mL/min. And then, continuously performing tangential flow circulation filtration for 40min to complete the purification of the silver nanowire stock solution, adding 15mL of 30 wt.% octadecyl dimethyl benzyl ammonium chloride into the purified solution, and stirring for 2h at 500rpm to perform secondary modification on the silver nanowires, thereby further improving the dispersibility and the stability of the silver nanowires. After finishing the modification, after the concentration of the purified silver nanowires is measured by adopting a drying mode, the silver nanowires are subjected to tangential flow filtration and concentration, and the silver nanowires are concentrated by adopting a last-stage tangential flow system, wherein the concentration of the concentrated silver nanowires is in a proper range of 0.15-0.3 wt.%, so that a silver nanowire concentrated solution is obtained.

The main experimental results are shown in table 1.

Comparative example 1

Purification of silver nanowires

Dispersing 2L of silver nanowire stock solution into 6L of deionized water, adding 200mL of ammonia water serving as a silver ion complexing agent, and stirring for 10min at the stirring speed of 500 rpm. Filtering the silver nanowire dispersion liquid by using a series three-stage tangential flow filtering device, wherein the interception aperture of a filtering column is 0.8 μm, 0.5 μm and 0.3 μm respectively, adding deionized water into a liquid storage tank during circulating filtration, wherein the replenishment speed is 100mL/min, purifying the silver nanowire stock solution after circulating filtration for 70min, performing concentration measurement on the purified silver nanowire in a drying mode, performing tangential flow filtration and concentration on the silver nanowire, concentrating the silver nanowire by using a last-stage tangential flow system, and obtaining a silver nanowire concentrated solution, wherein the concentration of the concentrated silver nanowire is in a proper range, generally 0.15-0.3 wt.%.

The main experimental results are shown in table 1.

Comparative example 2

Purification of silver nanowires

Dispersing 2L of silver nanowire stock solution into 6L of deionized water, adding 200mL of ammonia water serving as a silver ion complexing agent, and stirring for 10min at the stirring speed of 500 rpm. Filtering the silver nanowire dispersion liquid by using a series three-stage tangential flow filtering device, wherein the interception pore diameters of filtering columns are 0.8 mu m, 0.5 mu m and 0.3 mu m respectively, adding deionized water into a liquid storage tank while performing circulating filtration, wherein the replenishment speed is 100mL/min, and after filtering for 30min, adding a dispersion modifier while replenishing the deionized water, wherein the dispersion modifier added in the example is sodium polyacrylate, the concentration of the dispersion modifier is 10 wt%, and 100mL is added at one time. And (3) continuously performing tangential flow circulating filtration for 60min to complete the purification of the silver nanowire stock solution, performing tangential flow filtration and concentration on the purified silver nanowires after the concentration of the purified silver nanowires is measured in a drying mode, and concentrating the silver nanowires by using a last-stage tangential flow system, wherein the concentration of the concentrated silver nanowires is in a proper range, generally 0.15-0.3 wt%, so as to obtain a silver nanowire concentrated solution.

The main experimental results are shown in table 1.

Comparative example 3

Purification of silver nanowires

Dispersing 2L of silver nanowire stock solution into 6L of deionized water, adding 200mL of ammonia water serving as a silver ion complexing agent, and stirring for 10min at the stirring speed of 500 rpm. Filtering the silver nanowire dispersion liquid by using a series three-stage tangential flow filtering device, wherein the interception pore diameters of filtering columns are 0.8 mu m, 0.5 mu m and 0.3 mu m respectively, adding deionized water into a liquid storage tank while performing circulating filtration, wherein the replenishment speed is 100mL/min, and after filtering for 30min, adding a dispersion modifier while replenishing the deionized water, wherein the dispersion modifier added in the example is sodium polyacrylate, the concentration of the dispersion modifier is 10 wt%, and 100mL is added at one time. And (3) after circulating filtration is carried out for 60min, the raw solution of the silver nanowires is purified, 50mL of sodium polyacrylate with the concentration of 10 wt.% is added into the purified solution, and the mixture is stirred for 2h at 500rpm to carry out secondary modification on the silver nanowires, so that the dispersibility and the stability of the silver nanowires are further improved. After finishing the modification, after the concentration of the purified silver nanowires is measured by adopting a drying mode, the silver nanowires are subjected to tangential flow filtration and concentration, and the silver nanowires are concentrated by adopting a last-stage tangential flow system, wherein the concentration of the concentrated silver nanowires is in a proper range of 0.15-0.3 wt.%, so that a silver nanowire concentrated solution is obtained.

Comparative example 4

Silver nanowire purification

Dispersing 2L of silver nanowire stock solution into 6L of deionized water, adding 200mL of ammonia water serving as a silver ion complexing agent, and stirring for 10min at the stirring speed of 500 rpm. Filtering the silver nanowire dispersion liquid by using a series three-stage tangential flow filtering device, wherein the interception pore diameters of filtering columns are 0.8 mu m, 0.5 mu m and 0.3 mu m respectively, adding deionized water into a liquid storage tank while performing circulating filtration at a replenishment speed of 100mL/min, and after filtering for 30min, adding a ligand exchanger while replenishing deionized water, wherein the ligand exchanger added in the example is sodium borohydride with the concentration of 10 wt%, and 40mL is added at a speed of 4mL/min through an injection pump. And then, continuously performing tangential flow circulating filtration for 60min to complete the purification of the silver nanowire stock solution, performing tangential flow filtration and concentration on the purified silver nanowires by adopting a drying mode, and concentrating the silver nanowires by adopting a last-stage tangential flow system, wherein the concentration of the concentrated silver nanowires is in a proper range, generally 0.15-0.3 wt.%, so as to obtain a silver nanowire concentrated solution.

Preparation of a transparent conductive film by slurrying and measurement of photoelectric properties of the transparent conductive film under the same conditions as in example 1

The main experimental results are shown in table 1.

Comparative example 5

Silver nanowire purification

Dispersing 2L of silver nanowire stock solution into 6L of deionized water, adding 200mL of ammonia water serving as a silver ion complexing agent, and stirring for 10min at the stirring speed of 500 rpm. Centrifuging and re-dispersing the silver nanowire dispersion liquid for three times under the condition of 4000rpm for 20min, and adjusting the concentration of the silver nanowires to a proper range in the last time, wherein the concentration is generally 0.15-0.3 wt.%, so as to obtain the silver nanowire concentrated solution.

The photoelectric properties of the slurry and the transparent conductive film were measured under the same conditions as in example 1, and SEM images of the obtained silver nanowire stock solution and the purified concentrated solution are shown in fig. 10, which indicates that the purification effect is poor, and the purified silver nanowire solution still contains many silver nanoparticles and short rods. The main experimental results are shown in table 1.

Comparative example 6

And (3) silver nanowire purification: taking 1L of silver nanowire stock solution, dispersing the silver nanowire stock solution into 2L of deionized water, adding 50mL of ammonia water serving as a silver ion complexing agent, and stirring for 10min at the stirring speed of 500 rpm. Adding 9L of acetone into the silver nanowire dispersion liquid, standing for 3h, removing a supernatant after the silver nanowires are flocculated, adding 1.5L of a polyvinylpyrrolidone aqueous solution with the concentration of 0.5 wt.% and stirring for 1h for dispersion under the condition of 500rpm, repeating the flocculation dispersion step three times, finally centrifuging and re-dispersing for three times under the condition of 4000rpm 20min, and finally adjusting the concentration of the silver nanowires to a proper range, generally 0.15-0.3 wt.% to obtain the silver nanowire concentrated solution.

The photoelectric properties of the transparent conductive film prepared by slurrying and transparent conductive film were measured under the same conditions as in example 1, and SEM images of the obtained silver nanowire stock solution and the purified concentrated solution are shown in fig. 11, and it can be seen that the silver nanowires were broken after flocculation with acetone, and the breaking resulted in a decrease in the conductivity of the silver nanowires, and the main experimental results are shown in table 1.

TABLE 1 comparative table of experimental results of examples and comparative examples

As can be seen from the comparison of the results in table 1, the transparent conductive films of examples 1 to 6 have lower sheet resistance under the conditions of higher optical transmittance and lower haze than the comparative example. Especially a coating layer with the thickness of 10 mu m, the average haze is less than 1 percent, and the transmittance is more than 90 percent.

The foregoing is a more detailed description of the invention in connection with specific preferred embodiments and it is not intended that the invention be limited to these specific details. For those skilled in the art to which the invention pertains, several simple deductions or substitutions can be made without departing from the spirit of the invention, and all shall be considered as belonging to the protection scope of the invention.

Claims

1. A preparation method of silver nanowire solution is characterized by comprising the following steps: it includes:

step S1, synthesizing silver nanowires and diluting;

and step S2, circularly filtering and purifying the diluted silver nanowires by more than two stages of serially connected tangential flow filter columns, and adding a dispersion modifier for improving the Zeta potential and a ligand exchanger capable of reacting with water to generate active hydrogen in the filtering and purifying process.

2. The method for preparing a silver nanowire solution according to claim 1, characterized in that: in step S1, a polyol method is adopted to synthesize silver nanowires, and organic adsorbents are attached to the surfaces of the silver nanowires; the average diameter of the silver nanowires is not more than 100nm, and the average length of the silver nanowires is not less than 5 mu m.

3. The method for preparing a silver nanowire solution according to claim 2, characterized in that: the organic adsorbent is polyvinylpyrrolidone, and the molecular weight of the polyvinylpyrrolidone is 5000-30000000.

4. The method for preparing a silver nanowire solution according to claim 2, characterized in that: the interception aperture of the tangential flow filtration column is 50 nm-50 mu m, and the aperture size is reduced along with the increase of the stages.

5. The method for preparing a silver nanowire solution according to claim 2, characterized in that: in step S2, deionized water is used as a solvent during filtration; in the step S2, a solvent is supplemented in the filtering process, the supplementing speed is 1 mL/min-50L/min, and a dispersion modifier and a ligand exchanger for improving the Zeta potential are added while the solvent is supplemented.

6. The method for preparing a silver nanowire solution according to claim 2, characterized in that: the dispersion modifier comprises a positive dispersion modifier and a negative dispersion modifier, wherein the positive dispersion modifier comprises at least one of octadecyl amine hydrochloride, dioctadecyl amine hydrochloride, N-dimethyl octadecyl amine hydrochloride, dodecyl trimethyl ammonium chloride, hexadecyl trimethyl ammonium bromide, octadecyl dimethyl benzyl ammonium chloride and dodecyl dimethyl phenyl phosphorus bromide; the negative electricity dispersion modifier comprises at least one of sodium stearate, N-methyl amide carboxylate, ammonium lauryl sulfate, lauryl alcohol polyoxyethylene ether sodium sulfate, sodium lauryl sulfate, dodecylbenzene sulfonic acid, secondary alkyl sodium sulfonate, sodium polystyrene sulfonate, dodecyl phosphate, polyethylene glycol and sodium polyacrylate.

7. The method for preparing a silver nanowire solution according to claim 2, characterized in that: the ligand exchanger comprises at least one of borohydride, phosphite, hypophosphite, hypochlorite and hydrogen peroxide.

8. The method for preparing a silver nanowire solution according to claim 1, characterized in that: in step S1, the concentration of the diluted silver nanowire solution is 0.001 to 10 wt.%, and then the silver ion complexing agent is added and stirred for 0.001 to 100 hours to remove impurities in silver halide particles.

9. The method for preparing a silver nanowire solution according to claim 8, characterized in that: in step S1, the solvent used for dilution is one or a mixture of two or more of water, methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, 1, 2-butanediol, 1, 3-butanediol, n-pentanol, terpineol, ethyl acetate, propyl acetate, butyl acetate, dimethylformamide, and tetrahydrofuran;

in the step S2, more than two stages of tangential flow filter columns connected in series are adopted to circularly filter and purify the diluted silver nanowires, and the circulating filtration time is 0.001-200 h.

10. A method for preparing a transparent conductive film is characterized by comprising the following steps: which comprises the following steps:

preparing a silver nanowire solution by using the method for preparing the silver nanowire solution as claimed in any one of claims 1 to 9;

concentrating the filtered and purified silver nanowire solution to enable the concentration of the silver nanowires to be 0.002-10 wt.%;